The present invention relates to the technical field of electropyrotechnic initiators. More specifically, the invention relates to a process for the preparation of an electropyrotechnic initiator. This manufacturing process is suitable for any type of initiator. They are either detonating initiators, also known as detonators, or ignition-type initiators, intended for the ignition of propellant powder or of gas-generating substances. In the latter case, the most widespread application is that of the igniters for safety devices intended to protect the occupants of a motor vehicle.
An electropyrotechnic initiator is composed of a resistive heating element and of a heat-sensitive substance. The resistive element can exist in the form of a filament, of a small and very thin flat element deposited on a printed circuit support or of a resistive or semiconductor bridge made of thin layers.
When an electric current circulates in the resistive element, the latter is heated by the Joule effect and, for this reason, ignites the heat-sensitive substance. The latter must therefore be in close contact with the resistive element in order to provide for reliable transfer of heat between the resistive element and the heat-sensitive substance. This is generally obtained by exerting on the said substance, in the pulverulent state, a significant compressive force directed towards the resistive element and thus providing intimate contact and the maintenance thereof. Such a compressive process has disadvantages. This is because, in order to be capable of withstanding such a compressive force without being deformed, the structure of the initiator has to comprise very strong components, such as a compression ring, for example. Furthermore, the process for assembling the initiator requires the use of significant means for metering out and compressing the pulverulent substance. A distribution hopper and a hydraulic press are generally used. Such a process is generally carried out under a pressure of the order of 1 000 bar. Such a process, often employed in industry, thus requires significant means for protecting personnel against the risks inherent in the use and compression of dry explosive materials. Furthermore, during compression, the resistive element may be damaged.
In order to overcome the disadvantages of the compression techniques, described above, and to provide close contact of the pyrotechnic substance with the resistive element in a way which is stable and fixed over time, a person skilled in the art then attempted to xe2x80x9cadhesively bondxe2x80x9d the heat-sensitive substance to the resistive element. The heat-sensitive substance is then deposited in the form of a compact paint which adheres to the support.
A first method of preparation is to use solvents. Patent FR 2 704 944 and its corresponding U.S. Pat. No. 5,544,585 disclose such an embodiment. The explosive varnish disclosed in this patent is composed of a primary explosive or a heat-sensitive oxidizer-reducer mixture with the addition of 2 to 15% of film-forming binder, dissolved beforehand in a solvent. The varnish is deposited on the resistive element and the solvent is evaporated.
Another known preparation is that disclosed in Patent Application FR 2 794 235, filed under priority U.S. Pat. No. 0,927,5555. The explosive varnish comprises a pyrotechnic material in the form of particles and a binder composed of a particulate polymer resin. The varnish can additionally comprise a solvent, ethyl alcohol. The varnish is first of all deposited on the resistive element and then heated to a first temperature, of the order of 100xc2x0 C., to drive off the solvent, and then to a second temperature, of the order of 150xc2x0 C., to agglomerate the particles of the binder to one another and to bind the varnish to the resistive element.
There are disadvantages to these two methods of preparation. This is because, in a plant intended for large production volumes, the need to handle an explosive substance in the presence of volatile inflammable substances and to heat such substances to high temperatures represents a not insignificant restriction with regard to safety. Furthermore, toxic vapours can be produced during the removal of the volatile solvent.
Another known method of preparation for adhesively bonding the heat-sensitive substance to the resistive element is in situ polymerization. Patent FR 2 781 878 discloses such a method of preparation. The heat-sensitive substance comprises from 40 to 60% by weight of pulverulent pyrotechnic substance in suspension in 60 to 40% by weight of inert binder capable of curing by polymerization. The heat-sensitive substance is deposited on the resistive element and polymerization is obtained by heating or with radiation. In practice, this method of preparation requires high contents of binder. This therefore limits the proportion of pyrotechnic material in the heat-sensitive substance, increases its dispersion, and, for this reason, reduces the sensitivity of the initiator.
A person skilled in the art is therefore always on the look out for a process for preparing an electropyrotechnic initiator which does not present risks to the safety of the personnel and which makes it possible to deposit on and to adhesively bond to the resistive element a heat-sensitive substance in a way which is reliable and stable over time.
Such a process is a subject-matter of the present invention.
The invention relates to a process for the preparation of an electropyrotechnic initiator comprising a pyrotechnic varnish initiated by a resistive heating element, characterized in that the said pyrotechnic varnish is obtained by deposition, on the resistive element, of an aqueous adhesive composed of a dispersion of an explosive substance and of additives in an aqueous suspension based on a copolymer chosen from ethylene/vinyl acetate copolymers and vinyl acetate/ethylene acetate/ethylene copolymers and then by drying the said aqueous adhesive at a temperature of between 55xc2x0 C. and 75xc2x0 C. In the present application, the term xe2x80x9cvarnishxe2x80x9d refers to the solid component obtained by evaporation of the water present in the drop of aqueous adhesive deposited on the initiator. This varnish is also sometimes referred to as xe2x80x9cignition beadxe2x80x9d.
The invention exhibits the advantage of not employing volatile and inflammable solvents. This is because only water is used. In contrast to the preconceptions indicated in Patent FR 2 781 878, water is easily removed, the drying stage is not lengthy and the performance of the heat-sensitive substance is not damaged.
Another advantage of the invention is to produce a dispersion of the explosive substance in an aqueous suspension based on a copolymer. This makes it possible to render the explosive substance insensitive and to handle it without danger in the liquid state.
The aqueous adhesive is deposited in the liquid state on the resistive element in the form of a calibrated drop using a varnishing device comprising an air-operated metering device. The drop of aqueous adhesive is subsequently dried at a temperature of between 55xc2x0 C. and 75xc2x0 C., preferably at 60xc2x0 C. The fact of heating at a temperature of less than 80xc2x0 C. makes it possible to avoid the formation of bubbles and thus poor contact with the resistive element.
Drying is carried out according to a drying method standard in industry, such as infrared radiation, pulsed hot air or induction. Infrared radiation is a preferred drying method.
The explosive substance participating in the composition of the aqueous adhesive is chosen from the group consisting of primary explosives and oxidizer-reducer mixtures. According to a first preferred alternative form of the invention, the primary explosive is a dinitrobenzofuroxan salt and better still the primary explosive is potassium dinitrobenzofuroxane. According to a second preferred alternative form of the invention, the explosive substance is a mixture of zirconium and of potassium perchlorate.
This preparation process makes it possible to prepare lead-free aqueous adhesives. This is because the explosive substance participating in the composition can be devoid of lead, which makes it possible to prepare aqueous adhesives which are compatible with the environment.
The content of explosive substance in the pyrotechnic varnish after drying is between 65% and 95% by weight with respect to the total weight of the pyrotechnic varnish.
The water content of the aqueous adhesive before drying is between 55% and 70% by weight with respect to the total weight of the aqueous adhesive. This preparation process thus makes it possible to have, before drying, a high proportion of water, which facilitates the processing, and, after drying, a high proportion of explosive substance, which renders the initiator highly reactive.
According to a preferred alternative form of the invention, the copolymer-based aqueous suspension is obtained by emulsifying ethylene/vinyl acetate or vinyl acetate/ethylene acetate/ethylene copolymer in the presence of surfactant. The preferred surfactants are anionic surfactants and poly(vinyl alcohol). The amount of surfactant used is between 0.1% and 2% by weight with respect to the weight of the copolymer-based aqueous suspension.
According to another preferred alternative form of the invention, the copolymer-based aqueous suspension additionally comprises a plasticizer. This plasticizer is chosen from phthalates. Dibutyl phthalate is a preferred plasticizer. This compound makes it possible to improve the adhesion of the adhesive to the resistive element and to adjust the hardness of the adhesive. The amount of plasticizer is between 0% and 20% by weight with respect to the weight of the copolymer-based aqueous suspension.
Preferably, the copolymer is an ethylene/vinyl acetate copolymer and the proportion of ethylene is between 10 and 30% by weight with respect to the total weight of the ethylene/vinyl acetate copolymer. The amount of copolymer is between 50% and 60% by weight with respect to the total weight of the aqueous suspension based on said copolymer.
According to a preferred alternative form of the invention, the aqueous adhesive additionally comprises additives, such as a thickening agent and an X-ray marker.
The thickening agent is based on modified cellulose; this makes it possible to adjust the viscosity of the adhesive, between 6.5 and 9 Pa.s, to the deposition process. Mention may in particular be made, as thickening agent, of hydroxypropylcellulose, carboxymethylcellulose, methylhydroxypropylcellulose, hydroxyethylcellulose and carboxymethylhydroxyethyl-cellulose. The amount of thickening agent is between 0.5 and 2.5% by weight with respect to the total weight of the aqueous adhesive before drying.
Finally, the adhesive can also comprise an X-ray marker. Its role is to render the adhesive opaque to X-rays in the case where the other constituents of the aqueous adhesive, and in particular the explosive substance, do not comprise heavy metals. The X-ray marker is composed of a metal powder or a metal salt, the said metal having to sufficiently absorb X-rays while being compatible with the environment. It is preferably chosen from the group consisting of tungsten, zirconium, bismuth and silver. This marker thus makes it possible to monitor the initiator during its manufacturing process.
The fundamental novelty of the invention lies in the fact of using an aqueous adhesive comprising an explosive substance and a copolymer in suspension in water. The adhesive is deposited, using a varnishing device comprising an air-operated metering device, in the form of a calibrated drop on the resistive heating element and then the water is evaporated. The evaporation of the water and the nature of the copolymer make it possible to obtain, as shown in the tests carried out, very good adhesion of the adhesive to the resistive element.
The invention also relates to an electro-pyrotechnic initiator prepared according to the process described above comprising a pyrotechnic varnish initiated by a resistive heating element, characterized in that the said pyrotechnic varnish comprises:
from 60 to 95% by weight of explosive substance,
from 5 to 15% by weight of surfactant and of ethylene/vinyl acetate or vinyl acetate/ethylene acetate/ethylene copolymer,
from 0 to 25% by weight of additives.
The surfactant is chosen from anionic surfactants and poly(vinyl alcohol).
The additives comprise a thickening agent and an X-ray marker. The thickening agent makes it possible to adjust the viscosity of the adhesive; it is based on modified cellulose. The X-ray marker makes it possible to render the pyrotechnic varnish opaque to X-rays. It is a metal powder or a metal salt, the metal being chosen from the group consisting of tungsten, zirconium, bismuth and silver.
The explosive substance is chosen from the group consisting of primary explosives and oxidizer-reducer mixtures. A preferred primary explosive is a dinitrobenzofuroxan salt and better still potassium dinitrobenzofuroxane and a preferred oxidizer-reducer mixture is the mixture of zirconium and of potassium perchlorate.
Such an electropyrotechnic initiator operates with any type of resistive heating element. Preferably, the resistive heating element is a cylindrical filament, a bridge directly photoetched onto a printed circuit support or a bridge, made of thin layers, surface mounted on a printed circuit support.
According to a preferred alternative form of the invention, the explosive substance is a primary explosive and the resistive element is a semiconductor bridge, often denoted by the abbreviation SCB.